Experimental and theoretical studies were made of the energy and structure of= 3 tilt boundaries with <110> and <211> tilt axes. For both types of boundary, plots of the grain boundary energy versus the inclination of the boundary plane exhibited a minimum at high inclinations. The boundaries also exhibited complex 3-dimensional structures which featured a thin slab along the boundary plane within which the material adopted unusual crystal structures. In <110> tilt boundaries, the slab had a rhombohedral structure (9R) while, in the <211> tilt boundaries, it had a body-centered cubic structure. These structures seemed to be energetically favorable because they continued close-packed layers of atoms across the grain boundary.